The present disclosure relates generally to electronic circuits, and more particularly to an apparatus for detecting a power on condition utilizing a bandgap (BG) reference.
It is well known that electronic devices such as personal computers, televisions, digital cameras, personal entertainment devices, cellular phones, and similar others incorporate semiconductor integrated circuit (IC) chips, which are designed to operate at various power, voltage, and temperature levels. Many electronic devices use bandgap reference circuits that provide an accurate and reliable voltage reference that is stable over a wide operating temperature range. FIG. 1A is a diagram of a BG reference circuit 110, according to prior art. Differing current densities between matched transistors Q1-Q2 produces a delta Vbe voltage across resistor R3. An output bandgap reference voltage is generated by adding voltages across Q3 with the amplified delta Vbe voltage to produce a temperature invariant voltage reference.
During the startup and shutdown of these electronic devices the IC chips may enter an indeterminate operating state due to the unpredictable power supply voltages occurring during the startup and shutdown. For example, most IC chips require that a power supply provide a desired operating voltage before the IC chips are enabled to perform normal operation. Use of a power on reset (POR) circuit (which may also be referred to as a power on sense or a power on detection circuit) to insure presence of sufficient supply voltage is well known. The POR circuit senses a power on condition when a power supply voltage exceeds a desired trigger or threshold voltage level and provides a reset signal as an output that may be used to reset the IC chips to a known operating condition. The POR circuit may also be configured to detect abnormal operating conditions of the power supply including a brown out condition when a voltage level drops below the threshold.
The band gap of silicon at 300 K temperature is approximately 1.12 electron volts (eV) and for germanium it is 0.66 eV. The principle behind the bandgap reference is the well known voltage drop associated with certain semiconductor junctions. For example, a silicon p-n junction such as the emitter-base junction of a bipolar junction transistor (BJT) may have a forward conduction characteristic, e.g., a voltage drop of about 0.6 volts. It may be possible to construct a basic voltage reference circuit based on this known physical conduction property. For example, one or more such p-n junctions may be connected in series to form a voltage reference circuit that has a predetermined and stable output voltage such as by connecting two silicon diodes in series provides a regulated 1.2 volt output, by connecting three silicon diodes connected in series provide a regulated 1.8 volt output, and similar others. However, the voltage constructed by stacking diode voltages will not be temperature independent since it decreases with temperature, e.g., is complementary to absolute temperature (CTAT). As described earlier, the bandgap reference circuit 110 is typically designed to add voltages generated by a CTAT device to a proportional to absolute temperature (PTAT) device to obtain a voltage reference that is temperature invariant.
A voltage output of a conventional BG reference circuit is about 1.2 volts. FIG. 1B is a diagram of a BG referenced circuit referred to as a ‘Brokaw cell’ based POR circuit (BPOR) 100 according to prior art. The BPOR 100 deploys a total of 4 resistors (2 matched pairs) to provide a trip point for a supply voltage greater than 1.2 volts, the trip point voltage being adjusted by adjusting a ratio between two of the four resistors. However, the BPOR 100 consumes greater power and requires additional silicon area compared to the conventional BG referenced POR circuit that provides a trip point for a supply voltage of less than or equal to 1.2 volts. Therefore, a need exists to provide an apparatus for detecting a power on condition based on a BG reference that can provide a voltage output that changes in response to a supply voltage that is greater than 1.2 volts and preferably provide an improvement in power and silicon area compared to the BPOR 100.